The present invention relates in general to programmable parallel on-chip termination impedance and impedance matching, and more particularly, to parallel on-chip termination impedance and impedance matching circuits and methods.
Signal reflection on transmission lines can cause distortion that degrades the overall quality of the signal. To minimize signal reflection, transmission lines are resistively terminated. In the case of integrated circuits that are in communication with other circuitry on, for example, a circuit board, termination is often accomplished by coupling an external termination resistor (e.g., a 50 Ohm resistor) to the relevant input/output (I/O) pins.
A termination resistor is typically coupled to every I/O pin receiving an input signal from a transmission line. Often hundreds of termination resistors are needed for an integrated circuit. Numerous external termination resistors can consume a substantial amount of board space. The use of external components for termination purposes can be cumbersome and costly, especially in the case of an integrated circuit with numerous I/O pins.
It is therefore desirable to provide on-chip termination impedance to eliminate the need for the external components. It is further desirable to make such on-chip termination impedance insensitive to process, supply voltage, and temperature (PVT) variations.
The present invention provides efficient, flexible, and cost-effective on-chip parallel termination impedance. According to the present invention, termination transistors can provide parallel termination impedance to I/O pins on an integrated circuit. The impedance of the termination transistors is programmably adjustable. The impedance of the termination transistors can be programmed to meet different input/output standards.
The present invention also provides impedance matching circuitry. The impedance matching circuitry can match the impedance of the termination transistors to the impedance of external (off-chip) reference resistors. A user can change the impedance of the reference resistors to alter the impedance of the termination transistors. Thus, the impedance matching circuitry makes the termination transistors programmable.
A pair of external reference resistors can be used to control the parallel termination impedance provided to a large number of I/O pins on an integrated circuit. The present invention therefore eliminates the need for external components that are used to provide parallel termination impedance.